1. Field of the Invention
The present invention relates to a projection exposure apparatus, and more particularly to a projection exposure apparatus which can correct errors of orientations of images obtained from a plurality of projection optical units.
2. Related Background Art
Liquid crystal display panels are frequently used these days as a display device for word processors, personal computers, televisions, etc. The liquid crystal display panels are fabricated by patterning of transparent thin-film electrodes in a desired pattern on a glass substrate by the photolithography technique. As an apparatus for the lithography a mirror projection type aligner is used for exposure-printing an original pattern formed on a mask onto a photoresist layer on a glass substrate through a projection optical system.
Meanwhile, in order to enlarge the exposure area, the conventional mirror projection type aligners performed exposure while dividing the exposure area. Specifically, the exposure area on a plate as an exposed substrate is divided into four regions, and with scanning exposure of a first mask and a first region a circuit pattern of the first mask is transferred onto the first region. Then, exchanging the first mask for a second mask, the plate is moved in a step manner so that the second region comes to overlap with the exposure area of the projection optical system. Then performing scanning exposure of the second mask and the second region, a circuit pattern of the second mask is transferred onto the second region. After that, the same steps are repeated for a third mask and a third region and for a fourth mask and a fourth region, whereby a circuit pattern of the third mask or the fourth mask is transferred onto the third region or the fourth region, respectively.
In the case of the divisional exposure of the exposure area as described, the throughput (an amount of exposure substrate per unit time) is low because a plurality of scanning exposures are necessary. Further, there are seams between adjacent divided exposure regions and, therefore, the stitching accuracy must be enhanced. Thus, the magnification error of projection optical system needs to be decreased as close to zero as possible and a great improvement in alignment accuracy is required, thus increasing the costs of apparatus.
On the other hand, it can be conceivable that the scale of projection optical system be increased for full scanning exposure of a large exposure area without dividing the exposure area. However, in order to increase the scale of projection optical system, large-scale optical elements must be produced with very high accuracy, which would result in increasing the production cost and the size of apparatus. Also, the size increase of projection optical system would cause an increase in aberrations or degradation in imaging performance.